Test circuit and method

ABSTRACT

A test circuit includes an amplifier configured to receive an AC signal, and output an amplified AC signal based on the AC signal, a first detection circuit configured to generate a first DC voltage having a first value based on an amplitude of the AC signal, and a second detection circuit configured to generate a second DC voltage having a second value based on an amplitude of the amplified AC signal.

PRIORITY CLAIM

The present application claims the priority of U.S. ProvisionalApplication No. 62/948,014, filed Dec. 13, 2019, which is incorporatedherein by reference in its entirety.

BACKGROUND

Integrated circuits (ICs) often include circuits that generatealternating current (AC) signals and perform various functions involvingthe generated AC signals. AC signals have frequency values ranging fromless than one megahertz (MHz) to those corresponding to millimeter (mm)wavelengths. Properties of the circuits used to generate and performfunctions on the AC signals are sometimes susceptible to manufacturingprocess variations.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1A and 1B are schematic diagrams of a test circuit, in accordancewith some embodiments.

FIGS. 2A and 2B are schematic diagrams of oscillators, in accordancewith some embodiments.

FIGS. 3A and 3B are schematic diagrams of isolation circuits, inaccordance with some embodiments.

FIGS. 4A and 4B are schematic diagrams of amplifiers, in accordance withsome embodiments.

FIG. 5A is a schematic diagram of a detection circuit, in accordancewith some embodiments.

FIG. 5B is a depiction of detection circuit parameters, in accordancewith some embodiments.

FIGS. 6A and 6B are depictions of test circuit parameters, in accordancewith some embodiments.

FIG. 7 is a flowchart of a method of measuring an AC amplifier gain, inaccordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components, materials, values, steps,operations, materials, arrangements, or the like, are described below tosimplify the present disclosure. These are, of course, merely examplesand are not intended to be limiting. Other components, values,operations, materials, arrangements, or the like, are contemplated. Forexample, the formation of a first feature over or on a second feature inthe description that follows may include embodiments in which the firstand second features are formed in direct contact, and may also includeembodiments in which additional features may be formed between the firstand second features, such that the first and second features may not bein direct contact. In addition, the present disclosure may repeatreference numerals and/or letters in the various examples. Thisrepetition is for the purpose of simplicity and clarity and does not initself dictate a relationship between the various embodiments and/orconfigurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In various embodiments, a test circuit includes an oscillator configuredto generate an AC signal, an amplifier configured to output an amplifiedAC signal based on the AC signal, and first and second detectioncircuits. In operation, the first detection circuit generates a firstdirect current (DC) voltage having a first value based on an amplitudeof the AC signal, and the second detection circuit generates a second DCvoltage having a second value based on an amplitude of the amplified ACsignal. The test circuit is configured to provide electricalaccessibility to the first and second DC voltages in a DC testoperation, e.g., wafer acceptance testing (WAT), and the first andsecond values are usable to calculate a gain of the amplifier. The testcircuit thereby enables AC amplifier performance data to be obtainedusing DC test pads and equipment, which are smaller and lesscomplicated, respectively, than AC test pads and equipment, and withoutrequiring an AC test operation separate from a DC test operation.

FIGS. 1A and 1B are schematic diagrams of a test circuit 100, inaccordance with some embodiments. FIG. 1A is a block diagram of testcircuit 100 and FIG. 1B is a plan view of a semiconductor wafer 100Wincluding test circuit 100, in accordance with some embodiments.

Test circuit 100 is an IC that includes an oscillator 110, an isolationcircuit 120, an amplifier 130, a first detection circuit 140, and asecond detection circuit 150. Oscillator 110 includes an output terminal112 coupled to an input terminal 121 of isolation circuit 120, andisolation circuit 120 includes an output terminal 122 coupled to a nodeN1. In some embodiments, test circuit 100 does not include isolationcircuit 120, and output terminal 112 is coupled to node N1.

Amplifier 130 includes an input terminal 131 coupled to node N1, and anoutput terminal 132 coupled to a node N2 and to an output terminal100OUT. Detection circuit 140 includes an input terminal 141 coupled tonode N1, and an output terminal 142 coupled to a pad P1. Detectioncircuit 150 includes an input terminal 151 coupled to node N2, and anoutput terminal 152 coupled to a pad P2.

Two or more circuit elements are considered to be coupled based on adirect electrical connection or an electrical connection that includesone or more additional circuit elements, e.g., one or more logic ortransmission gates, and is thereby capable of being controlled, e.g.,made resistive or open by a transistor or other switching device.

Test circuit 100 also includes two or more input terminals (not shown)configured to receive one or more voltage levels, e.g., a power supplyvoltage level VDD, and a reference voltage level, e.g., a ground voltagelevel or power supply reference level VSS. In some embodiments powersupply voltage level VDD represents an operating voltage level, relativeto power supply reference level VSS, of amplifier 130, and each of theone or more voltage levels has a value, relative to power supplyreference level VSS, less than or equal to power supply voltage levelVDD. Each of oscillator 110, isolation circuit 120, amplifier 130, andfirst and second detection circuits 140 and 150 includes one or morenodes (not shown in FIGS. 1A and 1B) configured to carry the one or morevoltage levels and/or the reference voltage level.

Oscillator 110 is an electronic circuit configured to receive the one ormore voltage levels and the reference voltage level, and generate anoscillation signal VOSC on output terminal 112 in response to the one ormore voltage levels and the reference voltage level. Oscillation signalVOSC is an AC signal having a frequency ranging from about 10 megahertz(MHz) to about 100 gigahertz (GHz). In some embodiments, oscillator 110is configured to generate oscillation signal VOSC having a frequencyranging from 1 GHz to 10 GHz. In some embodiments, oscillator 110 isconfigured to generate oscillation signal VOSC having a frequencyranging from 24 GHz to 100 GHz. In some embodiments, an AC signal, e.g.,oscillation signal VOSC, is referred to as a radio frequency (RF)signal. In some embodiments, an AC signal, e.g., oscillation signalVOSC, having the frequency ranging from 24 GHz to 100 GHz is referred toas a mmWave signal.

In various embodiments, oscillator 110 is configured to generateoscillation signal VOSC having a single predetermined frequency orhaving one of multiple predetermined frequencies selectable responsiveto the one or more voltage levels and the reference voltage level.

In various embodiments, oscillator 110 includes one or more of a ringoscillator, a feedback oscillator, a differential oscillator, or othercircuit suitable for generating oscillation signal VOSC. In variousembodiments, oscillator 110 includes an oscillator 200A discussed belowwith respect to FIG. 2A or an oscillator 200B discussed below withrespect to FIG. 2B.

Isolation circuit 120 is an electronic circuit configured to receiveoscillation signal VOSC at input terminal 121 and, responsive tooscillation signal VOSC and one or more of the one or more voltagelevels and/or the reference voltage level, generate an AC signal VAC1 onoutput terminal 122, and thereby on node N1, having a same frequency asoscillation signal VOSC.

Isolation circuit 120 is configured to isolate oscillator 110 fromloading effects from one or both of input terminal 131 of amplifier 130being coupled to node N1 or input terminal 141 of detection circuit 140being coupled to node N1, and/or to isolate amplifier 130 and/ordetection circuit 140 from one or more frequency components ofoscillation signal VOSC. In various embodiments, isolation circuit 120includes one or more of a buffer, an inverter, a filter, or othercircuit suitable for isolating oscillator 110 from loading effectsand/or isolating amplifier 130 and/or detection circuit 140 from one ormore frequency components of oscillation signal VOSC. In variousembodiments, isolation circuit 120 includes an isolation circuit 300Adiscussed below with respect to FIG. 3A or an isolation circuit 300Bdiscussed below with respect to FIG. 3B.

In some embodiments in which test circuit 100 does not include isolationcircuit 120, oscillator 110 is configured to generate oscillation signalVOSC as AC signal VAC1 on node N1. In some embodiments, isolationcircuit 120 is integrated with oscillator 110, and oscillator 110 isthereby configured to generate AC signal VAC1 on node N1. In someembodiments, test circuit 100 does not include oscillator 110, andisolation circuit 120 is configured to receive oscillation signal VOSCfrom a circuit (not shown) external to test circuit 100. In someembodiments, test circuit 100 does not include oscillator 110 orisolation circuit 120, and is configured to receive AC signal VAC1 onnode N1 from a circuit (not shown) external to test circuit 100.

Amplifier 130 is an electronic circuit configured to receive AC signalVAC1 at input terminal 131 and, responsive to AC signal VAC1 and the oneor more voltage levels and the reference voltage level, generate an ACsignal VAC2 on output terminal 132, and thereby on node N2. Amplifier130 is configured to control an amplitude of AC signal VAC2 relative toan amplitude of AC signal VAC1, otherwise referred to as a gain ofamplifier 130, to have at least one predetermined target gain value. Insome embodiments, the gain of amplifier 130 is a ratio of the amplitudeof AC signal VAC2 to the amplitude of AC signal VAC1. In variousembodiments, the at least one predetermined target gain value includes avalue greater than, less than, or equal to one.

In various embodiments, amplifier 130 is configured to generate ACsignal VAC2 responsive to AC signal VAC1 having a single predeterminedtarget gain value or having one of multiple predetermined target gainvalues selectable responsive to the one or more voltage levels and thereference voltage level.

In various embodiments, amplifier 130 includes a push-pullconfiguration, a common-source configuration, or other arrangementsuitable for controlling an AC signal gain. In various embodiments,amplifier 130 includes an amplifier 400A discussed below with respect toFIG. 4A or an amplifier 400B discussed below with respect to FIG. 4B.

Each of detection circuits 140 and 150 is an electronic circuitconfigured to receive respective AC signal VAC1 or VAC2 at thecorresponding input terminal 141 or 151 and, responsive to thecorresponding AC signal VAC1 or VAC2 and the one or more voltage levelsand the reference voltage level, generate a corresponding DC signal VDC1on output terminal 142, and thereby on pad P1, or DC signal VDC2 onoutput terminal 152, and thereby on pad P2. Detection circuit 140 isconfigured to generate DC signal VDC1 having an amplitude that varies inresponse to variations in the amplitude of AC signal VAC1, and detectioncircuit 150 is configured to generate DC signal VDC2 having an amplitudethat varies in response to variations in the amplitude of AC signalVAC2.

In various embodiments, detection circuit 140 is configured to respondto an AC signal VAC1 amplitude increase by either increasing ordecreasing the DC signal VDC1 amplitude, and detection circuit 150 isconfigured to respond to an AC signal VAC2 amplitude increase by eitherincreasing or decreasing the DC signal VDC2 amplitude.

In various embodiments, detection circuits 140 and 150 have a sameconfiguration or different configurations. In various embodiments, oneor both of detection circuit 140 or 150 includes one or more gainstages, one or more low-pass filters, or other arrangement suitable forrepresenting an AC signal amplitude with a DC signal amplitude. Invarious embodiments, one or both of detection circuit 140 or 150includes a detection circuit 500 discussed below with respect to FIGS.5A and 5B.

In the embodiment depicted in FIG. 1A, output terminal 100OUT is an opencircuit such that detection circuit 150 is an entirety of a load atoutput terminal 132 of amplifier 130. In some embodiments, test circuit100 includes a load circuit (not shown) coupled to output terminal100OUT such that the load at output terminal 132 of amplifier 130includes detection circuit 150 and the load circuit. In someembodiments, a load circuit (not shown) external to test circuit 100 is(not shown) coupled to output terminal 100OUT such that the load atoutput terminal 132 of amplifier 130 includes detection circuit 150 andthe load circuit.

In the embodiment depicted in FIG. 1A, test circuit 100 includesoscillator 110, isolation circuit 120, and detection circuits 140 and150 arranged as discussed above as a built-in self-test circuit (BIST)100BIST configured to perform one or more test operations on adevice-under-test (DUT) 100DUT including amplifier 130. In someembodiments, test circuit 100 and BIST 100BIST do not include one orboth of oscillator 110 or isolation circuit 120 and are otherwiseconfigured as discussed above to perform one or more test operations onDUT 100DUT.

Test circuit 100 including BIST 100BIST is thereby configured to, inoperation, generate DC signals VDC1 and VDC2 having amplitudes usable tocalculate a gain of amplifier 130 of DUT 100DUT. Test circuit 100thereby enables AC amplifier performance data to be obtained using DCtest pads and equipment, which are smaller and less complicated,respectively, than AC test pads and equipment, and without requiring anAC test operation separate from a DC test operation.

FIG. 1B depicts an embodiment in which test circuit 100 is electricallyaccessible through pads P1-PN located at a top surface of semiconductorwafer 100W. In addition to semiconductor wafer 100W including pads P1-PNof test circuit 100, FIG. 1B depicts product dies D1-D4 and scribe linesSL between corresponding pairs of product dies D1-D4.

Scribe lines SL correspond to portions of semiconductor wafer 100Wbetween the product dies, e.g., product dies D1-D4, at whichsemiconductor wafer 100W is cut during a die separation process. In someembodiments, scribe lines SL have a width ranging from 80 micrometers(μm) to 120 μm.

Each of pads P1-PN is an exposed conductive layer, e.g., includingaluminum, copper, and/or another suitable metal, configured to provideelectrical accessibility, e.g., through a set of probe pins, tounderlying IC elements, e.g., output terminals 142 and 152 of testcircuit 100. In the embodiment depicted in FIG. 1B, each of pads P1-PNis electrically coupled to an element of test circuit 100. In someembodiments, one or more of pads P1-PN is electrically coupled to one ormore elements other than elements of test circuit 100, e.g., one or moreelements of a DC test structure or circuit (not shown).

Pads P1-PN have dimensions and spacing so as to be capable of inclusionin one or more scribe lines SL. In the embodiment depicted in FIG. 1B,pads P1-PN are arranged in a single column between product dies D3 andD4 have square shapes. In various embodiments, pads P1-PN are otherwisearranged and/or shaped so as to be capable of inclusion in one or morescribe lines SL. In some embodiments, pads P1-PN have square shapes withsides ranging from 20 μm to 80 μm. In some embodiments, pads P1-PN havesquare shapes with sides ranging from 30 μm to 50 μm.

In the embodiment depicted in FIG. 1A, test circuit 100 is configured togenerate DC signal VDC1 on pad P1 and DC signal VDC2 on pad P2 asdiscussed above. In various embodiments, test circuit 100 is configuredto generate one or both of DC signals on one or more pads other thanrespective pads P1 and P2. In various embodiments, test circuit 100 isconfigured to receive the one or more voltage levels and the referencevoltage level on two or more of pads P1-PN, e.g., pads P3-PN. In someembodiments, pads P1-PN have a number N ranging from 4 to 48. In someembodiments, pads P1-PN have the number N ranging from 8 to 24.

In the embodiment depicted in FIG. 1B, an entirety of test circuit 100,including pads P1-PN, is located in the scribe line SL between productdies D3 and D4. In various embodiments, some or all of test circuit 100is located outside of the scribe line SL in which pads P1-PN arelocated, e.g., in one or more adjacent scribe lines, product dies, ortest dies such as a process-control-monitor (PCM) die. In someembodiments an entirety of test circuit 100, including pads P1-PN, islocated in a test die, e.g., a PCM die.

By the configuration discussed above, e.g., the embodiment depicted inFIG. 1B, test circuit 100 provides electrical accessibility to DCsignals VDC1 and VDC2 in a DC test, e.g., WAT, operation, such that thefirst and second values are usable to calculate a gain of amplifier 130without requiring an AC test operation separate from a DC testoperation.

FIGS. 2A and 2B are schematic diagrams of respective oscillators 200Aand 200B, in accordance with some embodiments. Each of oscillators 200Aand 200B is usable as oscillator 110 discussed above with respect toFIGS. 1A and 1B.

Oscillator 200A includes output terminal 112 and nodes (not labeled)configured to carry power supply voltage level VDD and power supplyreference level VSS, each discussed above with respect to FIGS. 1A and1B, and PMOS transistors M1-M5 coupled in series with respective NMOStransistors M6-M10 between the power supply voltage level VDD andreference level VSS nodes. Gates and drain terminals of the transistorsof each transistor pair M1/M6-M5/M10 are coupled together, thetransistor pairs M1/M6-M5/M10 are coupled in series, and the drainterminals of the final transistor pair M5/M10 are coupled to the gatesof the first transistor pair M1/M6 and to output terminal 112 discussedabove with respect to FIGS. 1A and 1B. Oscillator 200A thereby includestransistors M1-M10 arranged as a ring oscillator configured to generateoscillation signal VOSC on output terminal 112, as discussed above withrespect to FIGS. 1A and 1B.

Oscillator 200A is configured to generate oscillation signal VOSC havingat least one frequency based on the ring oscillator configuration. Insome embodiments, oscillator 200A is configured to generate oscillationsignal VOSC having at least one frequency ranging from 1 GHz to 10 GHz.

In the embodiment depicted in FIG. 2A, oscillator 200A includes a totalof five transistor pairs M1/M6-M5/M10. In various embodiments,oscillator 200A includes a total of fewer or greater than fivetransistor pairs. In some embodiments, oscillator 200A includes one ormore switching devices (not shown), e.g., transistors, configured toswitchably control the total number of transistor pairs included in thering oscillator configuration, thereby switching between frequencies ofoscillation signal VOSC, in operation. In some embodiments, controlterminals, e.g., gates, of the one or more switching devices are coupledto one or more of pads P1-PN, discussed above with respect to FIGS. 1Aand 1B, and oscillator 200A is thereby configured to control a frequencyof oscillation signal VOSC responsive to one or more of the one or morevoltage levels discussed above with respect to FIGS. 1A and 1B.

Oscillator 200B includes nodes (not labeled) configured to carry powersupply voltage level VDD and power supply reference level VSS discussedabove with respect to FIGS. 1A and 1B. Between the power supply voltagelevel VDD and reference level VSS nodes, oscillator 200B includes aswitched resistor array RA1 coupled in series with an inductive deviceL1 and capacitive devices C1 and C2.

A transistor M11 is coupled in parallel with capacitive device C1, and adrain terminal of transistor M11 and terminals of each of inductivedevice L1 and capacitive device C1 are coupled together and to a firstterminal usable as output terminal 112 discussed above with respect toFIGS. 1A and 1B. A transistor M12 is coupled in parallel with capacitivedevice C2, and a drain terminal of transistor M12 and terminals of eachof inductive device L1 and capacitive device C2 are coupled together andto a second terminal usable as output terminal 112. A gate of transistorM12 is coupled to the first terminal, and a gate of transistor M11 iscoupled to the second terminal, transistors M11 and M12 thereby beingarranged in a cross-coupled configuration.

An inductive device, e.g., inductive device L1, is an IC structureconfigured to provide a targeted inductance value between two or moreterminals. In various embodiments, an inductive device includes a singleor multi-layer structure including one or more conductive, e.g.,metallic, segments, having a geometry suitable for providing a targetedinductance value. In some embodiments, an inductive device includes ameander line or a transmission line. In some embodiments, an inductivedevice includes a meander line or a transmission line positioned in ascribe line, e.g., a scribe line SL discussed above with respect toFIGS. 1A and 1B.

A capacitive device, e.g., capacitive device C1 or C2, is an ICstructure configured to provide a targeted capacitance value between twoor more terminals. In various embodiments, a capacitive device includesa plate capacitor, e.g., a MIM capacitor, a capacitor-configured MOSdevice, a variable capacitor, an adjustable capacitor, e.g., a MOSCAP,or another IC device suitable for providing a targeted capacitancevalue.

In the embodiment depicted in FIG. 2B, each of transistors M11 and M12is an NMOS transistor including a source terminal coupled to the powersupply reference level VSS node. In some embodiments, each oftransistors M11 and M12 is a PMOS transistor including a source terminalcoupled to the power supply voltage level VDD node.

By the configuration discussed above oscillator 200B is arranged as aninductor-capacitor (LC) resonator, including inductive device L1 andcapacitive devices C1 and C2, in cooperation with the cross-coupled pairof transistors M11 and M12. Oscillator 200B is thereby configured togenerate a differential AC signal (not labeled) at the first and secondterminals, either of which is usable as output terminal 112 with thecorresponding portion of the differential AC signal relative to powersupply reference level VSS being usable as oscillation signal VOSC.

Oscillator 200B is thereby configured to generate oscillation signalVOSC having a frequency based on the targeted inductance value ofinductive device L1 and the targeted capacitance values of capacitivedevices C1 and C2. In some embodiments, oscillator 200B is configured togenerate oscillation signal VOSC having a frequency ranging from 24 GHzto 100 GHz.

Switched resistor array RA1 includes resistive devices R1, R2, and R3,and switching devices SW1 and SW2 coupled in series with respectiveresistive devices R2 and R3. Resistive device R1 is configured inparallel with the series of switching device SW1 and resistive deviceR2, and in parallel with the series of switching device SW2 andresistive device R3.

A resistive device, e.g., resistive device R1, R2, or R3, is an ICstructure configured to provide a targeted resistance value between twoor more terminals. In various embodiments, a resistive device includes asingle or multi-layer structure including one or more conductive, e.g.,metallic, segments, having a geometry suitable for providing a targetedresistive value.

In the embodiment depicted in FIG. 2B, switched resistor array RA1includes a total of two switching devices SW1 and SW2 coupled in serieswith resistive devices R2 and R3. In various embodiments, switchedresistor array RA1 includes a total of fewer or greater than twoswitching devices coupled in series with resistive devices.

In the embodiment depicted in FIG. 2B, switched resistor array RA1 iscoupled between the power supply voltage level VDD node and inductivedevice L1. In some embodiments, e.g., embodiments in which oscillator200B incudes transistors M11 and M12 as PMOS transistors, switchedresistor array RA1 is coupled between inductive device L1 and the powersupply reference level VSS node.

Switched resistor array RA1 is thereby configured to, in operation,control current flow to the LC resonator including inductive device L1and capacitive devices C1 and C2, and cross-coupled transistors M11 andM12, such that the current is within a range within which oscillationoccurs. By being configured to control current flow as discussed above,switched resistor array RA1 enables tuning of oscillator 200B to addressmanufacturing process variations, in operation.

In some embodiments, control terminals of switching devices SW1 and SW2are coupled to circuit elements (not shown), e.g., non-volatile memorycells, and switching devices SW1 and SW2 are thereby configured to beswitched on or off responsive to a predetermined combination of powersupply voltage level VDD and power supply reference level VSS, inoperation. In some embodiments, control terminals of the switchingdevices, e.g., switching devices SW1 and SW2, are coupled to a subset ofpads P1-PN and the switching devices are thereby configured to beswitched on or off responsive to the one or more voltage levelsdiscussed above with respect to FIGS. 1A and 1B.

By including one of oscillators 200A or 200B configured to generateoscillation signal VOSC, test circuit 100 is capable of realizing thebenefits discussed above with respect to FIGS. 1A and 1B.

FIGS. 3A and 3B are schematic diagrams of respective isolation circuits300A and 300B, in accordance with some embodiments. Each of isolationcircuits 300A and 300B is usable as isolation circuit 120 discussedabove with respect to FIGS. 1A and 1B.

Isolation circuit 300A includes input terminal 121, output terminal 122,and the power supply reference level VSS node, each discussed above withrespect to FIGS. 1A and 1B, a switched resistor array RA2 coupledbetween input terminal 121 and output terminal 122, and a switchedcapacitor array CA1 coupled between output terminal 122 and the powersupply reference level VSS node. Switched resistor array RA2 andswitched capacitor array CA1 are thereby configured as a low-pass filtercapable of generating AC signal VAC1 on output terminal 122 based onoscillation signal VOSC received at input terminal 121, as discussedabove with respect to FIGS. 1A and 1B, by reducing harmonic componentsof oscillation signal VOSC, e.g., as provided by a ring oscillator suchas oscillator 200A.

Switched resistor array RA2 includes resistive devices R4 and R5, andswitching device SW3 coupled in series with resistive device R4.Resistive device R5 is configured in parallel with the series ofswitching device SW3 and resistive device R4. In the embodiment depictedin FIG. 3A, switched resistor array RA2 includes a total of oneswitching device SW3 coupled in series with resistive device R4. Invarious embodiments, switched resistor array RA2 does not includeswitching device SW3 coupled in series with resistive device R4 orincludes a total of greater than one switching device SW3 coupled inseries with resistive device R4.

Switched capacitor array CA1 includes capacitive devices C3 and C4, andswitching device SW4 coupled in series with capacitive device C4.Capacitive device C3 is configured in parallel with the series ofswitching device SW4 and capacitive device C4. In the embodimentdepicted in FIG. 3A, switched capacitor array CA1 includes a total ofone switching device SW4 coupled in series with capacitive device C4. Invarious embodiments, switched capacitor array CA1 does not includeswitching device SW4 coupled in series with capacitive device C4 orincludes a total of greater than one switching device SW4 coupled inseries with capacitive device C4.

In some embodiments, control terminals of the switching devices, e.g.,switching devices SW3 and SW4, are coupled to circuit elements (notshown), e.g., non-volatile memory cells, and switching devices SW3 andSW4 are thereby configured to be switched on or off responsive to apredetermined combination of power supply voltage level VDD and powersupply reference level VSS, in operation. In some embodiments, controlterminals of the switching devices, e.g., switching devices SW3 and SW4,are coupled to a subset of pads P1-PN and the switching devices arethereby configured to be switched on or off responsive to the one ormore voltage levels discussed above with respect to FIGS. 1A and 1B.

Switched resistor array RA2 and switched capacitor array CA1 are therebyconfigured to, in operation, control low-pass filtering characteristicssuch that harmonics of oscillation signal VOSC are reduced by apredetermined amount, thus enabling tuning of isolation circuit 300A toaddress manufacturing process variations.

Isolation circuit 300B includes input terminal 121, output terminal 122,the power supply voltage level VDD and reference level VSS nodes, eachdiscussed above with respect to FIGS. 1A and 1B, a resistive device R6coupled between input terminal 121 and output terminal 122, andtransistors M13 and M14 coupled between the power supply voltage levelVDD and reference level VSS nodes. Transistor M13 is a PMOS transistorincluding a source terminal coupled to the power supply voltage levelVDD node and a drain terminal coupled to output terminal 122. TransistorM14 is an NMOS transistor including a source terminal coupled to thepower supply reference level VSS node and a drain terminal coupled tooutput terminal 122.

Isolation circuit 300B thereby includes resistive device R6 andtransistors M13 and M14 arranged as an inverter, together configured asa push-pull circuit capable of isolating a circuit, e.g., oscillator110, coupled to input terminal 121 from loading effects from a circuit,e.g., amplifier 130 and/or detection circuit 140, coupled to outputterminal 122, as discussed above with respect to FIGS. 1A and 1B.

By including one of isolation circuits 300A or 300B configured togenerate AC signal VAC1 based on oscillation signal VOSC, test circuit100 is capable of realizing the benefits discussed above with respect toFIGS. 1A and 1B.

FIGS. 4A and 4B are schematic diagrams of respective amplifiers 400A and400B, in accordance with some embodiments. Each of amplifiers 400A and400B is usable as amplifier 130 discussed above with respect to FIGS. 1Aand 1B.

Amplifier 400A includes input terminal 131, output terminal 132, thepower supply voltage level VDD and reference level VSS nodes, eachdiscussed above with respect to FIGS. 1A and 1B, a resistive device R7coupled between input terminal 131 and output terminal 132, a transistorM15 and a switched PMOS array PA1 coupled between the power supplyvoltage level VDD node and output terminal 132, and a transistor M19 anda switched NMOS array NA1 coupled between output terminal 132 and thepower supply reference level VSS node.

Transistor M15 is a PMOS transistor including a source terminal coupledto the power supply voltage level VDD node, a drain terminal coupled tooutput terminal 132, and a gate coupled to input terminal 131.Transistor M19 is an NMOS transistor including a source terminal coupledto the power supply reference level VSS node, a drain terminal coupledto output terminal 132, and a gate coupled to input terminal 131.

Amplifier 400A thereby includes resistive device R7 and transistors M15and M19 arranged as an inverter, together configured as a push-pullcircuit capable of generating AC signal VAC2 on output terminal 132based on AC signal VAC1 received at input terminal 131 and a gain valuebased on conductance levels of transistors M15 and M19, as discussedabove with respect to FIGS. 1A and 1B.

Switched PMOS array PA1 includes switching devices SW5-SW7 coupled inseries with respective PMOS transistors M16-M18, each PMOS transistorM16-M18 being arranged in parallel with transistor M15. The switchingdevice/transistor pairs are thereby arranged as three parallel currentpaths between power supply voltage level VDD node and output terminal132 in addition to the current path provided by transistor M15. SwitchedNMOS array NA1 includes switching devices SW8-SW10 coupled in serieswith respective NMOS transistors M20-M22, each NMOS transistor M20-M22being arranged in parallel with transistor M19. The switchingdevice/transistor are thereby arranged as three parallel current pathsbetween output terminal 132 and power supply reference level VSS node inaddition to the current path provided by transistor M15.

Switching devices SW5-SW7 and switching devices SW8-SW10 are configuredto operate synchronously such that a number of parallel paths enabled inswitched PMOS array PA1 between power supply voltage level VDD node andoutput terminal 132 is equal to a number of parallel paths enabled inswitched NMOS array NA1 between output terminal 132 and power supplyreference level VSS node.

Because the gain value at which AC signal VAC2 is generated is based onthe conductance levels of each of switched PMOS array PA1 and switchedNMOS array NA1, amplifier 400A is thereby configured to generate ACsignal VAC2 having variable gain values determined by the number ofparallel paths enabled through switching devices SW5-SW7 synchronizedwith switching devices SW8-SW10. In operation, as the number of parallelpaths increases, the conductance level, and therefore the gain value,also increases.

In the embodiment depicted in FIG. 4A, each of switched PMOS array PA1and switched NMOS array NA1 includes a total of three switching devices,amplifier 400A thereby being configured to have four selectable gainvalues. In various embodiments, each of switched PMOS array PA1 andswitched NMOS array NA1 includes a total of fewer or greater than threeswitching devices, and amplifier 400A is thereby configured to havefewer or greater than four selectable gain values.

In some embodiments, control terminals of the switching devices, e.g.,switching devices SW5-SW10, are coupled to circuit elements (not shown),e.g., non-volatile memory cells, and switching devices SW5-SW10 arethereby configured to be switched on or off responsive to apredetermined combination of power supply voltage level VDD and powersupply reference level VSS, in operation. In some embodiments, controlterminals of the switching devices, e.g., switching devices SW5-SW10,are coupled to a subset of pads P1-PN and the switching devices arethereby configured to be switched on or off responsive to the one ormore voltage levels discussed above with respect to FIGS. 1A and 1B.

In some embodiments, amplifier 400A does not include switched PMOS arrayPA1 and switched NMOS array NA1, and amplifier 400A has a single gainvalue based on the conductance levels of transistors M15 and M19.

Amplifier 400B includes input terminal 131, output terminal 132, thepower supply voltage level VDD and reference level VSS nodes, eachdiscussed above with respect to FIGS. 1A and 1B, an inductive device L2coupled between the power supply voltage level VDD node and outputterminal 132, and a transistor M23 coupled between output terminal 132and the power supply reference level VSS node. A capacitive device C5 iscoupled between a gate of transistor M23 and input terminal 131, and aresistive device R8 is coupled between the gate of transistor M23 and anode (not labeled) configured to carry a gate voltage VG1.

Amplifier 400B is thereby arranged in a common-source configurationincluding NMOS transistor M23 as a gain stage, capacitive device C5 as aDC block element, inductive device L2 as a load element, and resistivedevice R8 as a bias element. In operation, gate voltage VG1 applied tothe gate of transistor M23 through resistive device R8 controls aconductance level of transistor M23, and thereby a gain of the gainstage and amplifier 400B. Amplifier 400B is thereby configured as acommon-source amplifier capable of generating AC signal VAC2 on outputterminal 132 based on AC signal VAC1 received at input terminal 131 andone or more gain values based on one or more values of gate voltage VG1.

In some embodiments, the gate voltage VG1 node is coupled to one or morecircuit elements (not shown), e.g., a switched resistor array, and theconductance level of transistor M23 is thereby configured to becontrolled responsive to one or more predetermined values of gatevoltage VG1. In some embodiments, the gate voltage VG1 node is coupledto a subset of pads P1-PN and the conductance level of transistor M23 isthereby configured to be controlled responsive to the one or morevoltage levels discussed above with respect to FIGS. 1A and 1B.

By including one of amplifiers 400A or 400B configured to generate ACsignal VAC2 based on AC signal VAC1, test circuit 100 is capable ofrealizing the benefits discussed above with respect to FIGS. 1A and 1B.

FIG. 5A is a schematic diagram of a detection circuit 500, in accordancewith some embodiments, and FIG. 5B is a depiction of detection circuit500 parameters, in accordance with some embodiments. Detection circuit500 is usable as one or both of detection circuits 140 or 150 discussedabove with respect to FIGS. 1A and 1B.

Detection circuit 500 includes an input terminal 141/151 usable aseither of input terminals 141 or 151, an output terminal 142/152 usableas either of output terminals 142 or 152, and the power supply voltagelevel VDD and reference level VSS nodes, each discussed above withrespect to FIGS. 1A and 1B. Detection circuit 500 includes a resistivedevice R10 coupled through a node N3 to a transistor M24, the seriescoupled between the power supply voltage level VDD and reference levelVSS nodes, and a resistive device R11 coupled through a node N4 to atransistor M25, the series coupled between the power supply voltagelevel VDD and reference level VSS nodes. A capacitive device C6 iscoupled between a gate of transistor M24 and input terminal 141/151, anda resistive device R9 is coupled between the gate of transistor M24 anda node (not labeled) configured to carry a gate voltage VG2. A resistivedevice R12 is coupled between node N4 and output terminal 142/152, and acapacitive device C7 is coupled between output terminal 142/152 and thepower supply reference level VSS node.

Resistive device R10, transistor M24, capacitive device C6, andresistive device R9 are thereby arranged in a common-sourceconfiguration including NMOS transistor M24 as a first gain stage,capacitive device C6 as a DC block element, resistive device R10 as aload element, resistive device R9 as a bias element, and node 3 as anoutput node. Resistive device R11 and transistor M25 are therebyarranged in a common-source configuration including NMOS transistor M25as a second gain stage in a cascade arrangement with the first gainstage, and node N4 as an output node. Resistive device R12 andcapacitive device C7 are thereby arranged in a low-pass filterconfiguration including node N4 as an input node and output terminal142/152.

In operation, gate voltage VG2 applied to the gate of transistor M24through resistive device R9 controls a conductance level of transistorM24, and thereby a gain value of the first gain stage, such that asignal VA is generated on node N3 based on the gain value and either ofsignals VAC1 or VAC2 discussed above with respect to FIGS. 1A and 1B(represented in FIGS. 5A and 5B as a signal VAC1/VAC2) received at inputterminal 141/151.

In operation, the second gain stage generates a signal VB on node N4based on signal VA and having a DC component that varies with theamplitude of signal VAC1/VAC2. The DC variation is based on a polarityof coefficients of even numbered harmonics of signal VAC1/VAC2 asamplified by the first and second stages. In some embodiments, thecoefficients are positive and the DC component of signal VB decreaseswith increasing VAC1/VAC2 amplitude. In some embodiments, thecoefficients are negative and the DC component of signal VB increaseswith increasing VAC1/VAC2 amplitude. In various embodiments, thepolarity of the coefficients is a function of one or both of a frequencyof signal VAC1/VAC2 or a gain value of the first gain stage based on avalue of gate voltage VG2.

In operation, the low-pass filter including resistive device R12 andcapacitive device C7 receives signal VB at node N4, and generates acorresponding one of signals VDC1 or VDC2 discussed above with respectto FIGS. 1A and 1B (represented in FIGS. 5A and 5B as a signalVDC1/VDC2) on output terminal 142/152 by attenuating the AC componentsof signal VB. In various embodiments, detection circuit 500 includes anarrangement other than that depicted in FIG. 5A and/or includes one ormore circuit components (not shown) in addition to resistive device R12and capacitive device C7 and thereby includes a low-pass filterconfigured to attenuate the AC components of signal VB.

Detection circuit 500 is thereby configured to receive one of signalsVAC1 or VAC2 at a corresponding input terminal 141 or 151, and generatea corresponding one of signals VDC1 or VDC2 at a corresponding outputterminal 142 or 152 and having an amplitude that varies with anamplitude of the corresponding signal VAC1 or VAC2. By the configurationdiscussed above, the amplitude of signal VDC1 or VDC2 either decreasesor increases with increasing amplitude of the corresponding signal VAC1or VAC2. In various embodiments, a relationship between the amplitudesof signal VDC1 or VDC2 and corresponding signal VAC1 or VAC2 varies as afunction of a frequency of the corresponding signal VAC1 or VAC2 and/oras a function of a value of gate voltage VG2.

In some embodiments, the gate voltage VG2 node is coupled to one or morecircuit elements (not shown), e.g., a switched resistor array, and thegain value of the first stage including transistor M24 is therebyconfigured to be controlled responsive to one or more predeterminedvalues of gate voltage VG2. In some embodiments, the gate voltage VG2node is coupled to a subset of pads P1-PN and the gain value of thefirst stage including transistor M24 is thereby configured to becontrolled responsive to the one or more voltage levels discussed abovewith respect to FIGS. 1A and 1B.

FIG. 5B depicts a non-limiting example of signals VAC1/VAC2, VA, VB, andVDC1/VDC2 of detection circuit 500, in accordance with some embodiments.Signal VAC1/VAC2, received at input terminal 141/151, is represented asan AC signal having a single frequency component at frequency f0. SignalVA, generated at node N3 by the first stage, includes a DC component, afirst component at frequency f0, a second component at frequency 2f0,and a third component at frequency 3f0. Each of the second component atfrequency 2f0, an even harmonic of frequency f0, and the third componentat frequency 3f0, an odd harmonic of frequency f0, has a positivecoefficient.

Signal VB, generated at node N4 by the second stage, includes a DCcomponent, a first component at frequency f0, a second component atfrequency 2f0, and a third component at frequency 3f0. Each of thesecond component at frequency 2f0, an even harmonic of frequency f0, andthe third component at frequency 3f0, an odd harmonic of frequency f0,has a positive coefficient.

Signal VDC1/VDC2, generated at output terminal 142/152, is representedas a DC signal with no significant AC signal components. An amplitude ofDC signal VDC1/VDC2 is based on an amplitude of AC signal VAC1/VAC2 atfrequency f0. In the non-limiting example depicted in FIG. 5B, based onthe positive coefficient of the second component of signal VB atfrequency 2f0, the amplitude of signal VDC1/VDC2 decreases as a functionof an increase in the amplitude of signal VAC1/VAC2 such that theamplitude of signal VDC1/VDC2 is usable to determine the amplitude ofsignal VAC1/VAC2.

By including detection circuit 500 configured to generate a DC signal,e.g., DC signal VDC1 or VDC2, based on an AC signal, e.g., AC signalVAC1 or VAC2, a test circuit, e.g., test circuit 100, is capable ofrealizing the benefits discussed above with respect to FIGS. 1A and 1B.

FIGS. 6A and 6B are depictions of test circuit 100 parameters, inaccordance with some embodiments. Each of FIGS. 6A and 6B includes plotsof simulated and measured gain values for each of four gain settings,with gain represented as a difference in DC signal amplitudes,VCD2−VDC1. For each gain setting, or mode, 0-3, a measured gain value Siis plotted along with simulated gain values for each of a fastmanufacturing process corner, FF_Sim, a typical manufacturing processcorner, TT_Sim, and a slow manufacturing process corner, SS_Sim.

In the non-limiting example depicted in FIG. 6A, test circuit 100includes oscillator 200A configured to generate oscillation signal VOSChaving a frequency of 4 GHz, isolation circuit 300A, and amplifier 400A.Gain modes 0-3 correspond to the number of enabled current paths ofswitched PMOS array PA1 and switched NMOS array NA1 of amplifier 400A.As indicated in FIG. 6A, the measured gain value Si for each gain modeis between the TT_Sim and SS_Sim gain values.

In the non-limiting example depicted in FIG. 6B, test circuit 100includes oscillator 200B configured to generate oscillation signal VOSChaving a frequency of 60 GHz, isolation circuit 300B, and amplifier400B. Gain modes 0-3 correspond to values of gate voltage VG1 applied tothe gate of transistor M23 of amplifier 400B. As indicated in FIG. 6B,the measured gain value Si for each gain mode is between the TT_Sim andSS_Sim gain values.

FIG. 7 is a flow chart of a method 700 of measuring an AC amplifiergain. Method 700 is capable of being performed with a test circuit,e.g., test circuit 100, discussed above with respect to FIGS. 1A and 1B.

The sequence in which the operations of method 700 are depicted in FIG.7 is for illustration only; the operations of method 700 are capable ofbeing executed in sequences that differ from that depicted in FIG. 7. Insome embodiments, operations in addition to those depicted in FIG. 7 areperformed before, between, during, and/or after the operations depictedin FIG. 7. In some embodiments, some or all of the operations of method700 are part of performing a DC test, e.g., a WAT, operation.

At operation 710, in some embodiments, a pad array of a semiconductorwafer is electrically accessed. Electrically accessing the pad arrayincludes contacting the pad array with a set of probe pins configured tohave a given probe pin make an electrical connection with acorresponding pad of the pad array. In some embodiments, electricallyaccessing the pad array includes electrically accessing the pad array inone or more scribe lines of the semiconductor wafer. In someembodiments, electrically accessing the pad array includes electricallyaccessing pads P1-PN discussed above with respect to FIGS. 1A and 1B.

In some embodiments, electrically accessing the pad array includesperforming an automated DC test, e.g., a WAT, operation by controllingmovement of the semiconductor wafer using an automated test system. Insome embodiments, electrically accessing the pad array of thesemiconductor wafer includes executing one or more software routines onthe automated test system. In some embodiments, electrically accessingthe pad array of the semiconductor wafer includes electrically accessingone pad array of a plurality of pad arrays of the semiconductor wafer.In some embodiments, electrically accessing the pad array of thesemiconductor wafer includes electrically accessing a number of padarrays of the semiconductor wafer ranging from four to ten.

In some embodiments, electrically accessing the pad array of thesemiconductor wafer includes electrically accessing the pad array of onesemiconductor wafer of a plurality of semiconductor wafers. In someembodiments, electrically accessing the pad array of the semiconductorwafer includes electrically accessing the pad arrays of a number ofsemiconductor wafers ranging from 20 to 30.

At operation 720, in some embodiments, an AC signal is generated. Insome embodiments, generating the AC signal includes generating an RFsignal having a frequency ranging from about 10 MHz to about 100 GHz.Generating the AC signal includes using an oscillator to generate anoscillator signal. In various embodiments, using the oscillator includesgenerating oscillator signal VOSC using oscillator 110 discussed abovewith respect to FIGS. 1A and 1B, oscillator 200A discussed above withrespect to FIG. 2A, or oscillator 200B discussed above with respect toFIG. 2B.

In some embodiments, generating the AC signal includes using anisolation circuit to generate the AC signal based on the oscillatorsignal. In various embodiments, using the isolation circuit includesgenerating AC signal VAC1 based on oscillator signal VOSC usingisolation circuit 120 discussed above with respect to FIGS. 1A and 1B,isolation circuit 300A discussed above with respect to FIG. 3A, orisolation circuit 300B discussed above with respect to FIG. 3B.

In some embodiments, generating the AC signal includes providing aplurality of DC voltage levels to the pad array. In some embodiments,generating the AC signal includes setting a frequency of the AC signalby providing one or more DC voltage levels of the plurality of DCvoltage levels to the pad array. In some embodiments, providing theplurality of DC voltage levels to the pad array includes providing theone or more voltage levels and the reference voltage level to padsP1-PN, as discussed above with respect to FIGS. 1A and 1B.

In some embodiments, providing the plurality of DC voltage levels to thepad array includes obtaining the plurality of DC voltage levels from astorage device using the automated test system.

At operation 730, an AC amplifier is used to generate an amplified ACsignal from the AC signal. In various embodiments, generating theamplified AC signal from the AC signal includes generating AC signalVAC2 from AC signal VAC1 using amplifier 130 discussed above withrespect to FIGS. 1A and 1B, amplifier 400A discussed above with respectto FIG. 4A, or amplifier 400B discussed above with respect to FIG. 4B.

Generating the amplified AC signal from the AC signal includes providinga plurality of DC voltage levels to the amplifier. In some embodiments,providing the plurality of DC voltage levels to the amplifier includesproviding one or more of the one or more voltage levels and thereference voltage level to pads P1-PN, as discussed above with respectto FIGS. 1A and 1B.

In some embodiments, generating the amplified AC signal from the ACsignal includes using the amplifier having a predetermined gain setting.In some embodiments, generating the amplified AC signal from the ACsignal includes setting a gain value of the amplifier. In someembodiments, setting the gain value includes setting switches SW5-SW10of amplifier 400A discussed above with respect to FIG. 4A. In someembodiments, setting the gain value includes providing gate voltage VG1to amplifier 400B discussed above with respect to FIG. 4B.

In some embodiments, setting the gain value includes providing aplurality of DC voltage levels to the pad array. In some embodiments,providing the plurality of DC voltage levels to the pad array includesproviding one or more of the one or more voltage levels and thereference voltage level to pads P1-PN, as discussed above with respectto FIGS. 1A and 1B. In some embodiments, providing the plurality of DCvoltage levels to the pad array includes obtaining the plurality of DCvoltage levels from a storage device using the automated test system.

At operation 740, a first DC voltage is output to a first pad, the firstDC voltage having a first value based on an amplitude of the AC signal.In some embodiments, outputting the first DC voltage to the first padincludes outputting signal VDC1 to one of pads P1-PN discussed abovewith respect to FIGS. 1A and 1B.

Outputting the first DC voltage includes outputting the first DC voltageusing a detection circuit. In various embodiments, outputting the firstDC voltage includes outputting DC signal VDC1 using detection circuit140 discussed above with respect to FIGS. 1A and 1B or detection circuit500 discussed above with respect to FIGS. 5A and 5B.

Outputting the first DC voltage includes providing a plurality of DCvoltage levels to the detection circuit. In some embodiments, providingthe plurality of DC voltage levels to the detection circuit includesproviding one or more of the one or more voltage levels and thereference voltage level to pads P1-PN, as discussed above with respectto FIGS. 1A and 1B.

In some embodiments, outputting the first DC voltage having the firstvalue based on an amplitude of the AC signal includes setting a gainvalue of the detection circuit. In some embodiments, setting the gainvalue includes providing a plurality of DC voltage levels to the padarray. In some embodiments, providing the plurality of DC voltage levelsto the pad array includes providing one or more of the one or morevoltage levels and the reference voltage level to pads P1-PN, asdiscussed above with respect to FIGS. 1A and 1B. In some embodiments,providing the plurality of DC voltage levels to the pad array includesobtaining the plurality of DC voltage levels from a storage device usingthe automated test system.

In some embodiments, outputting the first DC voltage includes using theautomated test system to store the first DC voltage in a storage device.

At operation 750, a second DC voltage is output to a second pad, thesecond DC voltage having a second value based on an amplitude of theamplified AC signal. Outputting the second DC voltage to the second pad,the second DC voltage having a second value based on an amplitude of theamplified AC signal, is analogous to outputting the first DC voltage tothe first pad, the first DC voltage having a first value based on anamplitude of the AC signal, as discussed above with respect to operation740, and a detailed description thereof is not repeated.

At operation 760, in some embodiments, an AC amplifier gain value iscalculated from the first and second values. In some embodiments,calculating the AC amplifier gain value includes calculating a ratio ofthe amplitude of the amplifier AC signal to the amplitude of the ACsignal.

In some embodiments, calculating the AC amplifier gain value includessubtracting the first value from the second value. In some embodiments,calculating the AC amplifier gain value includes comparing thecalculated AC amplifier gain value to a predetermined threshold value.In some embodiments, calculating the AC amplifier gain value includesusing the automated test system to store the calculated gain value in astorage device.

At operation 770, in some embodiments, one or more of operations 720-760is repeated. In various embodiments, repeating one or more of operations720-760 includes altering one or more of a frequency setting of anoscillator, a gain setting of an amplifier, or a gain setting of adetection circuit.

By executing some or all of the operations of method 700, DC test padsand equipment are used to obtain DC signals output by a test circuit andhaving amplitudes usable for determining a gain value of an AC signalamplifier, thereby obtaining the benefits discussed above with respectto test circuit 100 and FIGS. 1A and 1B.

In some embodiments, a test circuit includes an amplifier configured toreceive an AC signal, and output an amplified AC signal based on the ACsignal, a first detection circuit configured to generate a first DCvoltage having a first value based on an amplitude of the AC signal, anda second detection circuit configured to generate a second DC voltagehaving a second value based on an amplitude of the amplified AC signal.

In some embodiments, an IC includes an amplifier coupled between a firstnode and a second node, a first detection circuit coupled between thefirst node and a first pad positioned in a scribe line of asemiconductor wafer, and a second detection circuit coupled between thesecond node and a second pad positioned in the scribe line. Theamplifier is configured to generate an amplified AC signal at the secondnode based on a variable gain and an AC signal on the first node, thefirst detection circuit is configured to output a first DC voltage tothe first pad, the first DC voltage having a first value based on anamplitude of the AC signal, and the second detection circuit isconfigured to output a second DC voltage to the second pad, the secondDC voltage having a second value based on an amplitude of the amplifiedAC signal.

In some embodiments, a method of measuring an AC amplifier gain includesusing an AC amplifier to generate an amplified AC signal from a receivedAC signal, outputting a first DC voltage to a first pad of asemiconductor wafer, the first DC voltage having a first value based onan amplitude of the AC signal, and outputting a second DC voltage to asecond pad of the semiconductor wafer, the second DC voltage having asecond value based on an amplitude of the amplified AC signal.

It will be readily seen by one of ordinary skill in the art that one ormore of the disclosed embodiments fulfill one or more of the advantagesset forth above. After reading the foregoing specification, one ofordinary skill will be able to affect various changes, substitutions ofequivalents and various other embodiments as broadly disclosed herein.It is therefore intended that the protection granted hereon be limitedonly by the definition contained in the appended claims and equivalentsthereof.

What is claimed is:
 1. A test circuit comprising: an amplifierconfigured to receive an alternating current (AC) signal, and output anamplified AC signal based on the AC signal; a first detection circuitconfigured to generate a first direct current (DC) voltage having afirst value based on an amplitude of the AC signal; and a seconddetection circuit configured to generate a second DC voltage having asecond value based on an amplitude of the amplified AC signal.
 2. Thetest circuit of claim 1, further comprising an oscillator coupled inseries with an isolation circuit, wherein the isolation circuit isconfigured to output the AC signal based on an oscillation signal of theoscillator.
 3. The test circuit of claim 2, wherein the oscillatorcomprises a ring oscillator, and the isolation circuit comprises alow-pass filter.
 4. The test circuit of claim 2, wherein the oscillatorcomprises an inductor-capacitor (LC) resonator, and the isolationcircuit comprises a buffer.
 5. The test circuit of claim 1, wherein theamplifier comprises a variable-gain amplifier comprising a switched PMOSarray coupled to a switched NMOS array.
 6. The test circuit of claim 1,wherein the amplifier comprises a common-source configuration of atransistor and an inductive load.
 7. The test circuit of claim 1,wherein each of the first and second detection circuits comprises acascade arrangement of gain stages in series with a low-pass filter. 8.The test circuit of claim 1, wherein the first detection circuit iscapacitively coupled to an input terminal of the amplifier, and thesecond detection circuit is capacitively coupled to an output terminalof the amplifier.
 9. An integrated circuit (IC) comprising: an amplifiercoupled between a first node and a second node; a first detectioncircuit coupled between the first node and a first pad positioned in ascribe line of a semiconductor wafer; and a second detection circuitcoupled between the second node and a second pad positioned in thescribe line, wherein the amplifier is configured to generate anamplified alternating current (AC) signal at the second node based on avariable gain and an AC signal on the first node, the first detectioncircuit is configured to output a first direct current (DC) voltage tothe first pad, the first DC voltage having a first value based on anamplitude of the AC signal, and the second detection circuit isconfigured to output a second DC voltage to the second pad, the secondDC voltage having a second value based on an amplitude of the amplifiedAC signal.
 10. The IC of claim 9, further comprising an oscillatorcoupled to a third pad positioned in the scribe line, wherein theoscillator is configured to generate the AC signal on the first noderesponsive to a third DC voltage on the third pad.
 11. The IC of claim9, wherein the amplifier is coupled to a third pad positioned in thescribe line, and the amplifier is configured to control the variablegain based on a third DC voltage on the third pad.
 12. The IC of claim9, wherein the amplifier comprises an inductive load comprising ameander or transmission line positioned in the scribe line.
 13. The ICof claim 9, wherein each of the first and second detection circuitscomprises: a first common-source stage coupled to an input terminalthrough a capacitive device; a second common-source stage coupled to thefirst common-source stage; and a low-pass filter coupled between thesecond common-source stage and an output terminal.
 14. A method ofmeasuring an alternating current (AC) amplifier gain, the methodcomprising: using an AC amplifier to generate an amplified AC signalfrom a received AC signal; outputting a first DC voltage to a first padof a semiconductor wafer, the first DC voltage having a first valuebased on an amplitude of the AC signal; and outputting a second DCvoltage to a second pad of the semiconductor wafer, the second DCvoltage having a second value based on an amplitude of the amplified ACsignal.
 15. The method of claim 14, further comprising calculating again value from the first and second values.
 16. The method of claim 14,further comprising setting a gain of the AC amplifier by providing athird DC voltage to a third pad of the semiconductor wafer.
 17. Themethod of claim 14, further comprising generating the AC signal using aring oscillator or an inductor-capacitor (LC) resonator.
 18. The methodof claim 14, further comprising electrically accessing the first andsecond pads as part of performing a wafer acceptance test (WAT)operation on the semiconductor wafer.
 19. The method of claim 14,wherein the AC signal comprises a radio frequency (RF) signal, and theusing the AC amplifier to generate the amplified AC signal comprisesusing a push-pull circuit structure.
 20. The method of claim 14, whereinthe first and second pads are positioned in a scribe line of thesemiconductor wafer, the AC signal comprises a millimeter-wave signal,and the using the AC amplifier to generate the amplified AC signalcomprises using a common-source stage with an inductive load, theinductive load comprising a conductive line positioned in the scribeline.